1. Field of the Invention
This invention relates to electrolytic flotation systems for purifying waste water.
2. Background of the Related Art
The control of water pollution is an important objective of modern society. Millions of tons of waste water are produced daily in every city in the world. Industrial waste water has been of particular concern since it often contains metallic and oily residues which are both environmentally hazardous and difficult to remove.
One method of purifying waste water is by the introduction of very small bubbles ("microbubbles") into the waste water stream. The microbubbles adhere to particulates and other impurities, such as oils, in the waste water and lift them to the surface. The impurities can be removed by skimming a layer of froth from the surface of the water. There are many varieties of flotation systems, but most fall into one of two categories: the pressurized-air flotation systems and the electrolytic flotation systems.
In the pressurized-air flotation systems, air is introduced under pressure, causing it to dissolve in the waste water. The pressure is then relieved, and the air comes out of solution in the form of microbubbles. In other versions of the pressurized-air flotation systems, the air is introduced into the waste water at atmospheric pressure by a device which insures that the bubbles formed in the waste water are small. The small bubble size is important because small bubbles have more of a tendency to adhere to impurities. They are also more efficient, tending to float a greater mass of waste for a given mass of air.
By contrast to the pressurized-air flotation systems, the electrolytic flotation systems are considerably simpler in construction. They simply include several electrodes disposed in a tank through which the waste water flows. A potential difference is imposed between the electrodes, and the resulting current through the waste water causes electrolysis of the water to generate hydrogen and oxygen bubbles. The bubbles adhere to the impurities, causing flotation in a similar manner to that described in regard to pressurized-air flotation systems.
The electrolytic flotation systems are considerably simpler in design and thus require less of an initial investment. The electrolytic flotation systems do not require the compressors, high-pressure pumps, and pressuremonitoring devices that are required for pressurized-air flotation systems. The pressurized-air flotation systems, however, have been considerably more popular, because the electrolytic systems traditionally have required a high amount of energy resulting in high operating costs per unit mass of impurities removed from the waste water.
A typical electrolytic flotation system includes a treatment tank in which the waste water resides while the microbubbles attach themselves to impurities and carry them to the surface. The size of the flotation tank is determined by the capacity of the system--i.e., the number of gallons per minute of waste water that is treated by the system--and by the detention time, which is the average time required for the waste water to flow from the inlet to the outlet of the tank. The required volume of the tank is equal to its capacity multiplied by its detention time. For example, a flotation system that processes one gallon of waste water per minute and requires a detention time of one hour must have a volume of 60 gallons.
The capital cost of the system generally increases with its volume, so it would be desirable for the detention time to be as low as possible. A certain amount of detention time is required, however, in order to allow the microbubbles to mix into the waste water and diffuse through the fluid in the tank, adhere to the waste impurities and float to the surface. Practitioners in the art of electrolytic flotation believed that it is desirable to maintain a highly turbulent fluid flow, so that the bubbles and impurities would thereby quickly mix within the tank. For example, U.S. Pat. Nos. 3,944,478, 3,783,114 and 4,179,347, which are described in more detail below, utilize high speed, high power electrolysis, introducing the waste water to the electrodes in a turbulent flow. These systems are inefficient, in that they consume relatively large amounts of electrical power to produce a large volume of gas-per unit of impurities removed or volume of water processed.
The specific design parameters encountered in discussing the efficiency of electrolytic flotation systems are: the ratio of gas produced to solids removed; and, the ratio of energy consumed to the volume of water processed. The former parameter has ranged in prior systems, such as the ones listed above, from 0.3 to 0.06 or more kilograms of gas used per kilogram of solids removed, the ratio depending on the type of waste water and the intended level of purity. The gas-to-impurities ratio largely determines the energy required to float a given quantity of impurities, because, for a given electrode voltage, the power consumed is proportional to the rate of gas production.
Another problem inherent in electrolytic flotation systems, is the breakdown of the anodes due to oxidation, as well as, the build-up of scale, scum and dissolved minerals on the surface of the electrodes. Anodic breakdown, and scale and scum formation lead to loss of efficiency and a destruction of the electrodes, requiring replacement of the electrodes at frequent intervals. These additional problems translate into even higher operating costs.
Some electrolytic flotation systems take advantage of the anodic breakdown to introduce metallic ions into the waste water flow. Typical examples of these systems are disclosed in U.S. Pat. Nos. 4,194,172 and 3,944,678. Specifically, U.S. Pat. No. 4,199,972 to Weintraub et al., discloses breaking an oil-in-water emulsion by anodically dissolving ferrous ions into the emulsion. Likewise, U.S. Pat. No. 3,944,478 to Kuji et al., discloses an electrolytic drainage treating apparatus which includes a high speed, high power electrolyzer which includes a tank with a series of electrodes that are closely disposed perpendicular to the flow the waste water, an arrangement typical of prior electrolytic flotations systems.
The system described in the U.S. Pat. No. 3,944,478 uses an extremely high current and voltage source connected to the electrodes, freeing metallic ions from the anode, which eventually is consumed. The free ions attach to the particles of foreign matter in the waste water and the floc is then floated by the bubbles to the surface, where it is removed by a skimmer. This patent stresses the use of high speed, high power electrolysis in which a very fast and turbulent flow is required in order to clean the plates, and requires the use of a current density of over 1,000 amps per square meter (Col. 1, lines 44-52). Operation of this system, therefore, results in high energy costs per amount of waste removed, typically need in excess of 0.3 kg's of gas per kg of solids removed. In addition, the anodes must be replaced at regular intervals. In all such systems, the user is thus required to suffer the consequences of high operating costs for supplying energy to the electrodes and for replacement of the electrodes due to anodic breakdown.
Other electrolytic flotation systems take advantage of relatively inert materials such as lead and noble metals, such as platinum, for coating the anodes to prevent anodic destruction. However, the cost of noble metal coated electrodes significantly increases the initial cost of the flotation system. In addition, noble metal coated electrodes, as well as lead electrodes are still subject to scale and scum build up resulting in loss of efficiency and destruction of the coatings. Furthermore, the use of lead electrodes in waste water which may later be utilized for human consumption may lead to lead poisoning.
An electrolytic waste water treatment system which utilizes noble metal coating of the electrodes is disclosed in U.S. Pat. No. 4,179,347 to Krause et al., which provides an apparatus for chlorination and aeration of waste water using electrolytic flotation to separate solids from the water. This patent requires plates coated with precious metals in order to produce high speed, high powered electrolysis, and prevent rapid breakdown of the anodes. In addition, this patent requires the addition of an electrolyte such as sodium chloride to ensure high conductivity in the waste water for the high speed, high powered electrolysis to liberate chlorine gas in order to disinfect the waste water. The high speed liberation of bubbles in the waste water causes the agglomeration of the bubbles and reduces the efficiency of the device. In addition, the arrangement of baffles in the tank induce more turbulence and an upward streaming of the waste water around the bottom surface of the baffles. This arrangement increases the agglomeration and mixing of the bubbles, and thus, further reduces the efficiency of the electrolytic flotation system.
The use of precious metal coated electrodes in the system described in U.S. Pat. No. 4,179,347 does not solve the problem of scale and sludge build-up on the plates. Thus, this reference requires cleaning, scraping and skimming of the electrodes to remove the floated solids, and scale and scum adhering to the plates. The scraping of the plates, not only adds to the initial cost of the system for the additional scraping equipment, and to the operating cost of the system for the energy required to run the scrapers, but also wears down the precious metal coatings of the plates. The increased initial cost of this system due to the precious metal coated electrodes and the scraping and skimming apparatus, as well as the high operating costs due to inefficient high speed, high powered electrolysis, all have been contributing factors against the use of such electrolytic flotation systems.
The scrapers required in the electrolytic flotation system of U.S. Pat. No. 4,179,347 are typical of previous electrolytic and pressurized or dissolved air flotation systems. These designs use complex mechanical scrapers which pull the floated wastes from the top surface of the waste water, and in the case of electrolytic flotation systems, scrape floated wastes from the top surface of the electrodes, out of the tank onto a ramp. These scrapers are mechanically complex and require additional energy to remove the floated impurities. In addition, these methods not only remove floated impurities, but also introduce a high percentage of water which requires additional energy to remove by evaporation, incineration or landfill processes.
The technique of reversing the polarity of the electrodes at infrequent intervals has been utilized to remove scale and sludge build-up on noble metal coated electrodes. For example, U.S. Pat. No. 4,612,104 to Holmes et al., discloses an electrochemical cell for producing sodium hypochloride from a sodium chloride solution. The electrochemical cell is not directed towards electrolytic flotation, but rather, the sodium hypochloride is used for chlorination and disinfection of water. The cell utilizes annular platinum coated titanium electrodes, in a closed housing defining an annular flow passage in which a sodium chloride solution is introduced. Current reversal at infrequent intervals, approximately every 15 minutes, is used to remove calcium deposits from the platinum coated electrodes without damaging the platinum coating. At Col. 3, lines 9-12, this patent discloses that rapid current reversal for removing the calcium deposits would be detrimental, since it would damage the platinumcoated electrodes.
Other electrolytic flotation systems ignore anodic loss, and are more concerned with voltage build-up due to scale and slime formed on the electrodes. Such a system is disclosed in U.S. Pat. No. 3,783,114 to Ishii et al., in which relatively large iron anodes are suspended vertically, perpendicular to the flow of waste water; and, horizontally disposed cathodes are arranged close to the bottom of the tank. This arrangement prevents scum, arising from the electrolysis of the metal ions, fats and oils, from coating the cathodes. This system, however, requires a high current density and results in destruction of the anodes. For Column 3, lines 26 and 59-65. Accordingly, this systems requires the user to incur the cost of frequently replacing the anodes, as well as, high operating costs due to inefficient, high power electrolysis.
In the field of electrodialysis, which uses electrical potential differences for the separation and concentration of ions, scale and sludge formation present problems to the user. In electrodialysis systems, the electrical potential difference is not used for bubble formation to remove wastes by flotation, but rather the potential difference causes the migration of particles through a dialysis membrane. Scale and sludge build-up on the anode and dialysis membrane require frequent cleaning and replacement of these components. To solve this problem, U.S. Pat. Nos. 4,381,232; 4,578,160; 4,585,539; and 4,461,693 periodically reverse of the polarity of the electrodes to descale the anode and the dialysis membrane. Typical of these is U.S. Pat. No. 4,578,160 to Asano et al., describing a method for electrodialysis of dilute caustic alkali aqueous solutions. This patent discloses an electrodialysis cell divided by a cation exchange membrane with electrodes made from iron, nickel or their base alloys. In the cell, the current flows in the positive direction for as long as possible, usually about 15 minutes for approximately 97% to 60% of the total electrodialysis cycle. The current is then reversed for as short a duration as possible, typically 3% to 30% of the total electrodialysis cycle. Reversal causes sludge and scale to drop off the anode and cation exchange membrane (See Col. 5, lines 1-9, 10-19 and Col. 6, lines 8-13). This apparatus does not utilize electrolytic flotation and discloses at Col. 5, lines 1-9 that it is disadvantageous to rapidly reverse the direction of the current, over very short intervals of time. Due to the high current density, and the appreciable difference between the duration of current in the positive direction, as opposed to the duration of the current in the negative direction, this apparatus, as well as each apparatus disclosed in the other electrodialysis patents listed above, does not address, nor prevent anodic destruction.
Other, non analogous systems utilize infrequent intervals of current reversal in electrolytic solutions to deplete various electrolytes from the anode or cathode. For example, U.S. Pat. No. 4,632,737 to Mindler, discloses an apparatus for electrolytic reduction of nitrate from solutions of alkali metal hydroxides which are contaminated by oxidizing transition metal ions. The method utilizes the addition of bismuth to the solution at high current densities. The 15 ampere current is reversed at intervals ranging from about every two minutes to every 30 minutes in order to allow bismuth to be depleted from the cathode and bismuth petroxide to be depleted from the anode. This process results in anodic destruction due to high current densities and infrequent intervals of current reversal.
Accordingly, it is the object of the present invention to provide an electrolytic flotation system which efficiently removes contaminants from a waste water stream and has low electrical power requirements as compared to other electrolytic flotation systems.
It is another object of the present invention to provide an electrolytic flotation system which protects stainless steel electrodes from the effects of anodic destruction without requiring expensive precious metal coated electrodes.
A further object of the present invention is to provide electrolytic flotation system which protects the electrodes from scale and scum build-up and the attendant costs of maintenance and increased power consumption.
Still another object of the present invention is to provide a flotation system which efficiently removes floated wastes from the surface of the water without the use of scrapers or skimmers.